Sedimentary basins are prone to capture and amplify seismic energy. An important issue of concern for seismic hazard assessment is the amplification effect of the basin on the strong ground motion of earthquakes (Olsen, 2000). The knowledge of the 3D S wave velocity of the shallow structure, especially the quaternary sediments at 0-1 km, is required to understand the seismic response of sedimentary basins (Lai et al., 2020) as it is a main factor affecting the amplification. Meanwhile, unprecedented economic prosperity brought up the rapid development of large cities. As a result, one or more subcenters around the central city are constructed to meet the increasing population growth which cannot be carried by current city areas. High-resolution 3D shallow S wave velocity structure under the ground could provide a guideline for urban planning on earthquake prevention, it is therefore a key element in detailed geological survey before construction and is essential for disaster mitigation in many cities around the world (

Shear‐wave velocity (Vs) structures can reveal the shallow sediment thickness and deep tectonic features of buried faults and geological units. They are important for reducing seismic and geological disasters in urban areas. Based on ambient noise data from the Tongzhou dense array (919 seismographic stations), we obtain a fine shallow‐deep (0–5 km) 3D Vs model, by jointly inverting the phase‐velocity dispersions of Rayleigh waves, including short‐period (0.3–2 s) multimode dispersions using the frequency‐Bessel transform method and the long‐period (2–6 s) fundamental‐mode dispersions using the fast marching method. Our results show that the Vs inhomogeneities agree well with the distribution of geological units. We use the 1 km/s isodepth of Vs as the reference thickness of quaternary sediments. Fengbo sag (FBS) and Dachang sag (DCS), which mainly show low velocity and density, have thick sediment thicknesses (approximately 550–600 and 320–420 m, respectively). The NE high‐velocity belt in Daxing high (DXH) has a thinner sediment thickness (∼230 m). Thus, FBS and DCS have a greater risk of earthquake hazards owing to the strong amplification effects of ground motion. Additionally, Vs distribution in the FBS, DCS, DXH, and Yanshan Fold Belt are spatially related to the medium density and buried faults (Nanyuan‐Tongxian, Daxing, and Xiadian faults). We infer that the Vs structures are associated with the controlling effects of these large normal faults and inhomogeneous strata density. The discontinuity of the NE high‐velocity belt in DXH probably results from the intense tectonic activity of Nankou‐Sunhe fault.

Beamforming (BF) and Frequency-Bessel transform (F-J) have been demonstrated to extract multimode surface wave dispersion curves from ambient seismic noise. F-J method implicitly assumes the structure under the array is laterally isotropic. As for the conventional BF method, although the azimuth-dependence phase velocity can be measured, the fictitious azimuth anisotropy created by array geometry would be projected into the result. In this paper, the weighted and modified crosscorrelation beamforming (WCBF and MCBF) schemes are proposed to extract the multimode surface wave dispersion curves with sufficient resolution using quite short noise recordings. Compared with the conventional BF, only the plane waves with the azimuth consistent with the interstation orientations are considered in MCBF and the search over the incident plane waves from different azimuth is omitted. The azimuth-dependence velocity can therefore be extracted by MCBF, independent of the array geometry. As far as the measurement of azimuth-averaged velocity is concerned, we show that BF is equivalent with F-J. The explicit relationship between BF and F-J methods is derived. For the finite sampling in practical applications, the theoretical representations of the dispersion image generated by BF technique under different imaging conditions are given. These representations can be used to investigate analytically the features of the dispersion images in frequency-velocity domain and how the aliasing is eliminated by improved imaging condition. The proposed methods are validated for the synthetic data as well as the real data from the dense array at different scales.